Rest Your Fears: Big Earthquakes Not on the Rise

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SAN FRANCISCO — While Earth seems to be getting slammed with
frequent mega- earthquakes lately, big quakes are not on the
rise.

That's the message from two studies presented here this week at
the annual meeting of the American Geophysical Union. Two
research teams using different statistical methods both found
that the
global risk of big earthquakes is not higher than usual.
Neither team found any evidence that big
earthquakes can trigger other big earthquakes over long
distances.

"We tend to see patterns in random processes, that's just
something we do," said Andrew Michael, a U.S. Geological Survey
scientist who presented his work Wednesday (Dec. 7). "In
particular, people expect when something's random for it to be
uniformly spread out, but, in fact, really random processes have
a lot of clustering in them."

That clustering can make it look like there are patterns in the
short term, Michael said, even when the long-term statistics
don't show any meaningful variation.

The rate of big earthquakes

On a local level,
earthquakes do cluster and trigger one another, with a main
shock often surrounded by fore- or aftershocks. But whether large
earthquakes that occur thousands of miles across the globe from
each other are related is a separate question.

In research presented Monday (Dec. 5), University of California
San Diego geophysicist Peter Shearer and UC Berkeley statistician
Philip Stark reported that the recent rate of magnitude-7.5 to
magnitude-8 quakes is close to its historical average. Since
2004, magnitude-8 quakes have been more common than usual, the
researchers reported, but this blip is consistent with normal
variation, the researchers reported.

Such giant earthquakes are expected to occur at least once during
the 111-year history of the catalogue of quake data, they said.

Random patterns

In a second study, the USGS's Michael used three statistical
methods to find out if large earthquakes cluster together or if
what looks like clusters is just random variability. A first
glance at
global earthquakes since 1900 does look very clustered, he
said. But as soon as you remove aftershocks from the equation,
that pattern disappears.

"That tells us that all the clustering we were seeing on the
global scale was just an effect of local clustering," Michael
told LiveScience.

Michael also looked at time periods after a large quake to see if
other large quakes peaked in the following months and years.
Again after removing direct aftershocks, he found no such
evidence. A third test again failed to uncover evidence of
clustering.

"Really, if you take any data set and you look for patterns in it
and you insist on things happening in a very similar way, things
will always look very surprising," he said. "Even in random
sequences you can sort of define yourself into a corner where
things seem unique."

The risk of earthquakes hasn't gone down either, Michael warned,
and people living near areas where large quakes have hit should
keep their guard up. Aftershocks to giant quakes like the
March 2011 Tohuko quake in Japan can be very large
themselves, he said.